Mechanism of below-threshold high-order harmonic generation of Na in the intense elliptically polarized laser field

We explore the mechanism of below-threshold harmonic generation (BTHG) of Na atom subject to intense elliptically polarized laser fields. The Na atom is described by an accurate model potential which reproduces both the bound state energies and the oscillator strengths. An intuitive picture is demonstrated by the analysis of the quantum channels of electron in BTHG. The HHG spectra can be calculated accurately and efficiently by solving three-dimensional time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. The results show that the yields of the BTHG generated by the elliptically polarized laser fields are high when the ellipticity is decreased, and the yields of the BTHG are sensitive to the ellipticity of the laser fields. Our results provide a potential method to control the enhancement of the BTHG by the polarized laser field.